This invention relates to a flow meter utilizing a Karman vortex street.
Flow meters utilizing the Karman vortex street include a vortex generating rod immersed in a fluid flowing through a conduit perpendicularly to the direction of flow of the fluid to generate the Karman vortex street downstream of the rod and employ an ultrasonic wave to detect the Karman vortex street thereby to measure a flow rate of the fluid. In order to detect the Karman vortex street, it has been proposed to dispose an ultrasonic transmitter and an ultrasonic receiver in opposed relationship in the conduit through which a measured fluid flows so that an ultrasonic wave transmitted from the ultrasonic transmitter is modulated by the vortices of the Karman vortex street and then received by the ultrasonic receiver. Under these circumstances the ultrasonic wave is propagated through the fluid flowing through the conduit until the same reaches, in addition to the ultrasonic receiver, that portion of the inner wall of the conduit opposite to the transmitter. Then the ultrasonic wave is reflected from the inner conduit wall and interferes with that propagated from the transmitter resulting in the formation of a standing ultrasonic wave. Alternatively, the ultrasonic wave might be reflected from the inner conduit wall a few times and then be received, as noise, by the ultrasonic receiver resulting in the disadvantage that the measurement cannot be stably made. Particularly, when the flow rate of air is being measured, the ultrasonic wave is propagated through the air at a propagation velocity which changes with a change in the air temperature. If the ultrasonic wave being used has a constant frequency, then this change in propagation velocity is attended by a variation in the wavelength thereof. This has resulted in the disadvantage that a standing ultrasonic wave is formed at a certain temperature of the air which causes the ultrasonic receiver not to receive in a normal fashion the ultrasonic wave transmitted from the ultrasonic transmitter.
Since the ultrasonic wave transmitted by the ultrasonic transmitter is propagated through the flowing fluid as above described, if there is a long distance between the ultrasonic transmitter and receiver, this is disadvantageous in that the ultrasonic receiver is not only faint but also drifts with the flowing fluid so as to be attenuated.
Accordingly, it is an object of the present invention to provide a new and improved flow meter utilizing a Karman vortex street to measure the flow rate of a fluid over a wide range and at temperatures extending from a low to an elevated temperature.